Numerous reports highlighted the importance of Mycoplasmae spp. [1 - 6] and the deregulation of the 2.5A synthetase RNase L antiviral pathway [9 - 14] in subsets of Chronic Fatigue Syndrome (CFS). We hypothesised there may be a co-morbid physiopathological mechanism between infection by Mycoplasma species and the deregulation of the 2,5A synthetase / RNase L antiviral pathway in Chronic Fatigue Syndrome. The study was conducted in Brussels, at a university-based outpatient clinic (Vrije Universiteit Brussel). We enrolled 192 consecutive patients seeking care for prolonged fatigue as major complaint between the first of January and the end of June 1999, who complied with the Fukuda et al [7] definition. All subjects had to be free of antibiotic-treatment two months prior to blood collection. Mycoplasmae-infected CFS-patients presented with significantly elevated RNase L-ratio, compared to non-infected subjects. These results suggest a strong interaction between Mycoplasmae infections and a deregulation of the 2,5A synthetase RNase L antiviral pathway. Indeed, Mycoplasmae are active in stimulating several components of the immune system. They can act as polyclonal T-cell and B-cell activators [17], and some Myoplasmas can trigger macrophages in vitro [15]. To bring about their phagocytic activity, monocytes produce elastase. The latter enables them to pass through connective tissues. Elastase is capable of cleaving 80 kDa RNase L [16], thus causing deregulation of the antiviral pathway. In addition, low molecular weight RNase L has been suggested to reduce TH1 activity [8]. The latter implicates an increased susceptibility to infections and a suppressed ability to eliminate intracellular antigens.